1. Field of Invention
This invention relates to a novel, environmentally acceptable process for recycling waste lignocellulosic material to produce a highly delignified pulp of acceptable strength and cleanliness; the invention relates particularly to a process of recycling which reduces the discharge of environmental pollutants.
2. Description of Prior Art
The manufacture of paper or paperboard generally involves the digestion of wood chips by the kraft pulping process to produce a lignocellulosic pulp mass which is comprised of two main components; a cellulosic component and a lignin component. The cellulosic component largely comprises the wood fiber while the lignin component is more concentrated between the fibers as a structural element binding wood fibers together. However, a substantial portion of the lignin is also distributed within the fibers themselves.
The kraft pulping process produces a dark colored mass of lignocellulose fibers commonly known as "brownstock". The dark color is due to the presence of chemically altered lignin which contains chromophoric groups. Further delignification processes such as oxygen delignification or chlorination are performed followed by bleaching processes to make a white colored pulp.
A brownstock pulp produced directly from the digestion of wood chips is generally referred to as a virgin pulp. In contrast, a brownstock pulp produced from the repulping of waste paperboard is generally referred to as a recycle pulp. In both cases, the residual lignin content of a brownstock pulp is indicated by its kappa number. Higher kappa numbers indicate higher residual lignin contents.
The kappa number of a brownstock pulp obtained from cooking softwood is typically 50 to 100, and such a pulp is used for making the linerboard component of corrugated paperboard. The kappa number of a brownstock pulp from cooking hardwood is typically 130 to 160, and such a pulp is used for making the corrugated medium component of corrugated paperboard. Corrugated paperboard waste comprising linerboard and corrugating medium generally has an overall kappa number of 80 to 120.
The kappa number of a softwood brownstock pulp would need to be reduced to about 25 to 35, and that of a hardwood brownstock to about 10 to 15 to provide a pulp suitable for bleaching to produce white paper products. However, the removal of lignin or delignification usually results in the degradation of the cellulose component as well. The relative difference between the extent of lignin degradation and cellulose degradation is commonly referred to as "selectivity".
A number of processes for delignifying pulp with oxygen have been proposed, such as Richter U.S. Pat. Nos. 1,860,432 (1932), Gaschke et al. 3,274,049 (1966), and Farley 3,251,730 (1972). The use of oxygen as a delignification agent, however, has not been a completely satisfactory one since it is less selective when compared to other agents such as elemental chlorine. Furthermore, the kappa number reduction that can be attained with oxygen delignification is limited to a level beyond which attack on cellulosic fibers becomes disproportionate.
Improved selectivity at a higher degree of delignification has been the object of numerous proposals to improve oxygen as a delignification agent. These include multi-stage oxygen treatment, alkaline pretreatment, use of hydrogen peroxide, and others. Oxygen delignification is usually carded out at medium consistency because high consistency operation typically results in the poorest chemical selectivity while low consistency operation typically results in the poorest delignification efficiency.
Alkaline pretreatment of pulp prior to oxygen delignification has been suggested by Elton in U.S. Pat. Nos. 4,806,203 (1989), and by Terrell et al. in 5,173,153 (1992). The use of multiple consecutive oxygen bleaching stages with interstage countercurrent washing has been proposed by Prough in U.S. Pat. No. 4,946,556 (1990).
The use of oxygen and alkaline pretreatment permits the recycling of effluent back to the kraft recovery boiler when mixed with the residue remaining after digesting wood chips. This residue, commonly known as black liquor, is collected, concentrated by means of evaporation, and then incinerated in a high temperature boiler, commonly known as a recovery boiler. This process provides a means for the recovery of energy and chemical value from the black liquor. The methods and apparatus used for recovery of pulping chemicals from black liquor is conventional and well known in the art.
The concentration and subsequent incineration of black liquor typically limits the pulp production rate in a kraft pulp mill. In order to substantially increase pulp production or to recover oxygen stage effluent, the evaporator systems and the recovery boiler often require modifications or replacement to handle the additional solids flow increase. Since the recovery operation is a source of a substantial proportion of a pulp mill's discharge of air pollutants, pollution abatement equipment must be upgraded as well.
The utilization of oxygen delignification and other associated alkaline pretreatment has the disadvantage of increasing the amount of black liquor residue that must be processed by a pulp mill recovery system. A substantial increase in black liquor residue typically requires modification of such systems as described above and thereby increases the potential level of air pollutants that may be emitted from the recovery boiler. Capital costs for these modifications involve millions of dollars of new investment.
Abrahamsson described a brownstock pretreatment with nitrogen dioxide to enhance the oxygen delignification process in "Oxygen/Sodium Carbonate Bleaching of Kraft Pulp Pretreated with Nitrogen Dioxide and Oxygen", Svensk Papperstidning (1983). Selectivity improvement was the object of Meier et al. in proposing a pretreatment with peroxomonosulfuric acid in U.S. Pat. No. 5,091,054 (1992).
A process for enhancing the bleachability of kraft brownstock is taught by du Manoir et al. in U.S. Pat. No. 5,179,021 (1993). This process involves a series treatment of oxygen delignification followed by xylanase enzyme treatment prior to a chlorination stage in a typical bleach plant. Such a process provides for a delignified and bleached pulp using lower amounts of chlorine containing compounds than previously taught in the prior art. However, xylanase pretreatment prior to an oxygen treatment stage has not been observed to substantially enhance oxygen delignification.
The use of hydrogen peroxide to brighten and delignify lignocellulosic pulps is well known in the art and widely used in commercial bleach plant installations. Its use has typically been limited to treatments of low kappa kraft pulps for bleaching because of its high chemical cost and because of its poorer selectivity in comparison to oxygen, chlorine, and chlorine dioxide.
The poorer selectivity of hydrogen peroxide is generally thought to result from decomposition reactions catalyzed by soluble metals and by elevated temperature. Hydrogen peroxide decomposes into oxygen and water with increasing pH, temperature, and heavy metal concentrations. Intermediate decomposition products are also produced, including radicals such as HO.degree. and HOO.degree., which lead to lower yields and strength by oxidation and degradation of lignin and cellulosic fibers. Stabilizing chemicals such as sodium silicates and chelants are well known additives used to reduce the decomposition rate of hydrogen peroxide.
The use of hydrogen peroxide in a two stage oxygen delignification process is proposed by Parthasarathy et al. in U.S. Pat. No. 5,011,572 (1991). The patent describes a method to improve the oxygen selectivity and degree of delignification of chemical pulp by the addition of hydrogen peroxide.
Hydrogen peroxide has also been widely used in brightening white pulp made from deinked wastepaper. Deinked wastepaper pulps are typically contaminated with residual inks which lower the visual brightness of a recycled pulp. Use of hydrogen peroxide for this purpose has grown substantially in recent years due to the increasing recovery of waste paper and paperboard products.
The total recovery of paper and paperboard for recycling purposes has been projected to increase to 49 million tons by the year 2000 as a result of the need to reduce the volume of waste paper and paperboard being landfilled. The recovery of kraft paperboard, particularly old corrugated containers, has increased substantially with an estimated 184% increase in recovery between the years 1985 and 1992.
Because of its high kappa number and high level of contaminants, waste paperboard has not been used for white paper manufacture. Its recovery has been typically limited to producing paperboard containing recycle fiber.
Several processes have been proposed or implemented for the recovery of kraft paperboard. Such processes generally involve the repulping of used paperboard, cleaning the resulting pulp of nonwoody contaminants, and mixing the cleaned pulp with virgin kraft pulp for the purpose of making kraft linerboard or corrugating medium components of corrugated paperboard. In general, the decontamination steps used in the art are not highly stringent since the recycle pulp is typically reused in paperboard. Substantial delignification steps are not required for recycling waste paperboard into paperboard products.
Recycle pulp fiber is usually mixed with a virgin pulp because the recycle fiber's strength is inferior due to its prior processing history. The strength differences between recycle and virgin pulp fibers are often substantial. This typically limits the recycle content in white papers to 20% by weight or less. Because of it's higher weight, paperboard products can utilize higher levels of recycle fiber.
In practice, some paperboard producers subject the recycle pulp to alkaline soaking to enhance the strength characteristics and to reduce the yield losses associated with screening as described by Kohler in "Is Japanese Technology Right for Recycled Containerboard in North America?", Recovery and Reuse of OCC (1993). However, one object of the treatment is to limit pulp delignification in order to minimize associated yield losses. The recycle pulp is often further subjected to treatments of starch and resins to reinforce the paperboard strength.
Modifications of the kraft cooking process have been proposed for treating waste paperboard for the purpose of delignifying the pulp to provide an improved product. In Canadian Pat. No. 1,110,411 (1981), Moore describes a process whereby wax coated or resin impregnated paperboard is repulped in an immersion of weak kraft black liquor at a temperature of 65.degree. C. to 93.degree. C. to enhance the repulping characteristics of the paperboard and to facilitate the separation of wax and resins from the fibers. Key to the process is the objective to separate the wax by melting it at elevated temperature. A disadvantage of the process is that a portion of the dissolved wax material will deposit back onto the pulp fibers as the pulp mixture cools. The deposited wax then inhibits bonding between fibers and acts to discolor the pulp in localized areas. Wax coatings, long used as an effective moisture barrier for packaging products, causes quality problems in recycle pulps. Therefore, waxed corrugated board is not an acceptable source of recycle fiber.
In U.S. Pat. No. 5,147,503 (1992), Nguyen describes a process for recycling waste paperboard which includes the digestion of paperboard in an alkaline cooking liquor, recovery of spent cooking chemicals, and the bleaching of the cooked pulp for the manufacture of white paper or paperboard. Nguyen's process is obviously very similar to the kraft pulping process and therefore retains disadvantages associated with kraft pulping such as high capital cost, process complexity, and the discharge of air pollutants. The kraft like processes described above require the utilization of sodium sulfide which leads to the generation and release of noxious sulfur compounds from the system. Furthermore, the economic viability of such processes are generally dependent on the integration into operations which have chemical recovery systems in place.
Oxygen delignification has been proposed for treating waste paperboard for the purpose of delignifying recycle pulp to provide an improved product. In U.S. Pat. No. 4,737,238 (1988), recycling of paper products containing aluminum is described by de Ruvo in which the screened pulp is subjected to oxygen delignification. In U.S. Pat. No. 5,302,244 (1994), Nguyen describes a process for recycling waste cellulosic paperboard which comprises subjecting paperboard to a pretreatment with sulfuric acid and then subjecting the recycle pulp to multiple stages of medium consistency oxygen delignification to produce a pulp with a kappa number of 15 to 35. As with previous processes, these methods rely on costly evaporation and incineration technologies to treat effluent streams.
Despite the research conducted in the area of pulp preparation, and despite the fact that paper recycling has been practiced for many years, the substantial recovery and reuse of paper and paperboard is a relatively recent phenomena. The art related to this practice is not well advanced so new processes for converting waste materials into improved quality products are needed. This is evident by the highly variable quality of existing recycle pulps as described by W. B. Darlington, "Comparative Properties of Market Deinked Pulps", Tappi Pulping Conference, pg 741 (1992).
Recycle pulps are generally weaker and more contaminated with extraneous materials than virgin pulps. Furthermore, because of prior processing, the drainage characteristics of recycle pulps are poorer in relation to virgin pulps. While these disadvantages present functional problems in the final paper product, they also create problems in the manufacturing process as well. The slower drainage characteristics and poorer strength of recycle pulps often require a paper machine to operate at a substantially lower production speed than it would otherwise run on virgin pulp. The contaminants present in the pulp generally result in defects in the paper which cause, upon substantial tension as in a paper making machine, initiation of a rapture or break of the paper thereby causing the machine to stop production. In printing operations, such contaminants can stick to printing blankets and cause the printing operation to cease.
The discharge of waste streams to the environment is a problem not satisfactorily treated in the prior art. This is of particular importance due to increasing regulatory pressure for expanded waste water treatment. In general, most proposed recycling processes rely on existing paper mill waste water treatment systems to treat collected waste water prior to discharge. Others rely on existing chemical recovery systems to evaporate and incinerate these waste streams. However, many pulp mills are limited in their ability to further process waste water or additional black liquor and therefore require substantial modification or replacement. Additional solids loading to such recovery systems typically increases the amount of air pollutants emitted from the process and therefore must be permitted for operation by regulatory agencies. In some cases, permits at higher air pollutant emissions are denied. A stand alone recycle process which recovers and reuses all liquid filtrate streams in an economical manner has yet to be developed and would therefore represent a significant advancement in the art.
Of particular problem in the recovery and reuse of liquid waste streams is the build up of organic and inorganic dissolved solids. Excessive build up of dissolved solids results in scaling and corrosion of process equipment and can affect the chemistry of bleaching and delignification. For example, excessive metal ions can decompose hydrogen peroxide into radicals which nonselectively react with cellulosic fibers.
Numerous treatment processes have been proposed to treat water for reuse in processing plants. Most conventional approaches involve the concentration of waste streams through evaporation and incineration to produce a solid ash residue. However, this is cosily and presents problems associated with ash disposal and the emission of air pollutants. Other technology well known in the art include ultrafiltration, reverse osmosis, ion exchange, electrolysis, crystallization, biological treatment, and chemical precipitation. In general, the application of such technologies is costly with high operating and maintenance costs. Furthermore, concentrated waste streams are produced which must be further disposed of.
The National Council of the Pulp and Paper Industry for Air and Stream Improvement (NCASI) commissioned "A Preliminary Engineering Study and Cost Implication of Convening Deinking Mills to Closed Cycle Operations" NCASI Special Report No. 94-03 (March 1994). In the study, a panel of industry experts identified a feasible waste water treatment scheme to accomplish complete closure of a recycle pulp plant which included primary and secondary biological treatment, filtration, microfiltration, reverse osmosis, evaporation, crystallization, incineration, and ash fixation. Ion exchange was not selected as a feasible treatment method. The total installed capital for treating the waste water from a hypothetical 300 ton per day recycle plant was $38 to $44 million dollars with operating and maintenance costs of $2.6 to $4.2 million dollars per year. The NCASI report illustrates the high cost and complexity associated with mill closure and demonstrates how those highly skilled in the art would approach this industry recognized problem.